TypeError: You provided 'undefined' where a stream was expected

In my case I was returing with empty return:

if (...)
   return; // problem here

To fix, I returned the observed object:

if (...)
   return req.next();

For me, I had a subscription piped with takeUntil like this:

this.route.queryParams
  .pipe(takeUntil(this.ngUnsubscribe))
  .subscribe(async(queryParams) => {
  });

I forgot to initialize this.ngUnsubscribe with an observable. When I set the value to an observable in the component (ngUnsubscribe = new Subject()), the error was gone.

I faced this issue when creating a new project using generator-jhipster-ionic (yo jhipster-ionic giving v3.1.2), following Matt Raible's (OP) Use Ionic for JHipster to Create Mobile Apps with OIDC Authentication blog article, but choosing JWT authentication instead of OIDC.

The origin of the issue was a mix of problems.

I had the issue when running a Ionic app with the livereload server, CORS issues happening and Angular HTTP returning the classic HTTP failure response for (unknown url): 0 Unknown Error, where 0 is the HTTP error code. However, in the current case the issue was hidden by bad error handling at the Observable level.

When you follow Angular's HttpClient #Error Details section advices, and add an Observable pipe with a catchError operator on the HTTP POST call, you can get the proper HTTP error details. In this case, instead of going down to the rxjs/util/subscribeToResult.js:71 (TS source #L80) TypeError: You provided 'undefined' where a stream was expected default error case, it goes to rx/util/subscribeToResult.js:23 (TS source #L34), and the error is handled properly in the piped method.

After following the Observable calls, I found that the current default authentication interceptor, as seen in this template src/providers/auth/auth-interceptor.ts catches HTTP error 401 and does nothing for the others, basically muting them and preventing their propagation.

TL;DR In the JWT case, the solution is to simply remove the src/providers/auth/auth-interceptor.ts .catch(...) block, allowing error propagation to login.service.ts, and in its this.authServerProvider.login(credentials).subscribe((data) => { ... }, (err) => { ... }) error callback.

I believe the issue and solution could be the same for your OIDC case, its signup method, and error callback.

[Edit] Even more since the same .catch code can be found in the starter example mentioned in the first post comments: ionic-jhipster-starter - auth-interceptor.ts#L31